Acute lymphoblastic leukemia (ALL) is a disease induced by the transformation of blood (hematopoietic) stem cells and progenitors. It mainly afflicts children and adolescents. Although treatment outcome in ALL has improved in recent years, patients with relapsed disease continue to have dismal prognosis. It is thus very important to understand the molecular pathways that control both induction of transformation and treatment resistance in this cancer type. Recent studies demonstrated that the majority of T-cell ALL (T-ALL) patients show signs of hyperactivation of the Notch signaling pathway. Also, the leukemic cells appear to be addicted to Notch signaling. Excessive Notch activity is either due to gain-of-function Notch1 mutations or loss-of-function mutations of the ubiquitin ligase Fbw7 (SEL- 10) which is able to bind and induce the degradation of nuclear oncogenic Notch. Identical, inactivating, Fbw7 mutations were also recently found in a significant fraction (approximately 10%) of several types of solid tumors including breast, colon and gastric carcinomas suggesting that Fbw7 is a key tumor suppressor. To prove this hypothesis, we have conditionally deleted the Fbw7 locus and shown that hematopoietic progenitor-specific loss of Fbw7 predisposes the animals to T cell leukemia and lymphoma due to the stabilization of Notch1 and pathway hyperactivation. All these observations underline the importance of the Notch-Fbw7 interaction in cancer, and more specifically in leukemia. They also suggest that regulation of this interaction could prove to be a powerful therapeutic tool for the treatment of the disease. In this application we propose experiments that will identify novel regulators of the Notch-Fbw7 regulatory interaction and test their ability to modulate Notch pathway activation in leukemia using animal models of the disease. To achieve that, we will use a powerful RNAi-based in vivo screen using the C.elegans as our model organism. Genes, putative regulators of the Notch:Fbw7 interaction will be then tested in mammalian in vitro and in vivo screens as well as mouse models of T-ALL. PUBLIC HEALTH RELEVANCE: The interaction between the oncogene Notch1 and the tumor suppressor Fbw7 is crucial for the induction of blood malignancies and solid tumors. In this application we are identifying genes that can regulate the Notch:Fbw7 interaction using a powerful C.elegans RNAi-based screen. Genes identified in this screen are further tested in both human cancer and animal models of Notch-induced disease.